<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <button>点击</button>
  <script>
    const btn = document.querySelector('button')
    function fn() {
      console.log('我是回调函数A');
    }

    function fn2() {
      console.log('我是回调函数B');
    }
    btn.addEventListener('click', fn)
    btn.addEventListener('click', fn2)


    // 消除这个事件fn2
    btn.addEventListener('click',fn2)


  </script>
</body>

</html>